The present invention relates in general to a flywheel, and more particularly to improvements of a flywheel having an outer ring gear portion.
An engine such as an internal combustion engine operated by combustion of a fuel is widely used as a drive power source for automobiles, for example. Such an engine inevitably suffers from a periodic variation in operating speed (angular velocity) or torque in synchronization with a combustion cycle. To reduce or minimize this variation, a flywheel is usually attached to a crankshaft which is rotated by the engine. A flywheel is also used to reduce a periodic variation in the operating speed of other rotating members not associated with an engine.
DE 44 03 306 discloses an example of a known two-piece flywheel used for automobiles, which consists of an inner body and a separate ring gear. Another type of known two-piece flywheel is manufactured by press-fitting or shrink-fitting an inner body into a separate ring gear, as indicated in FIG. 7. Generally, the ring gear is formed from a carbon steel blank (e.g., S48C blank) usually used for machine structures, by hobbing or other machining operation using a pinion cutter, for example. The ring gear has a multiplicity of teeth formed along the periphery of a ring member such that the teeth extend over the entire axial dimension of the ring member, namely, ring gear. On the other hand, the inner body has a central attaching section for attachment to a crankshaft, and is generally formed by casting of a gray iron such as FC230. The inner body is press-fitted or shrink-fitted into the opening of the ring gear to produce the flywheel. That is, the flywheel consists of the inner body member and the outer ring gear which are fixed to each other.
Where the flywheel is used with an engine, the outer ring gear of the flywheel is held in meshing engagement with a pinion of an engine starter motor, so that the crankshaft is rotated by the starter motor to start the engine.
The known flywheel the inner body of which is formed of a cast gray iron tends to suffer from insufficient tensile strength at its attaching section. In view of this drawback, the attaching section is conventionally required to have a large wall thickness in the axial direction of the flywheel. However, this solution results in an increase in the weight of the inner body member, and a decrease in the ratio of the weight of the outer ring gear to the total weight of the flywheel. Accordingly, the moment of inertia of the flywheel is reduced, leading to an undesirable increase in the amount of variation in the rotary motion of the crankshaft, and a consequent decrease in the fuel economy of the engine.
A further drawback of the known flywheel is a relatively high cost of manufacture, primarily because of the use of two blanks, which must be processed to produce the ring gear and the inner body member, respectively, before these two members are assembled together into the flywheel. Thus, there is a limitation in the reduction of the manufacturing cost of the known flywheel.
The conventional flywheel also has a problem of insufficient mechanical strength (insufficient toughness, for example) due to hardening treatment effected during the manufacture, while the flywheel is generally required to have a sufficiently high degree of durability at a high operating speed. In this respect, it is noted that the engine of a modem vintage is required to have a relatively high maximum operating speed.
It is therefore a first object of the present invention to provide a flywheel which has an increased strength at its outer ring gear portion and which is economical to manufacture.
It is a second object of this invention to provide a method of economically manufacturing a flywheel having an increased strength at its outer ring gear portion.
The first object indicated above may be achieved according to a first aspect of the present invention, which provides a flywheel which is generally disc-shaped and which includes a ring gear disposed along a periphery thereof and has a friction surface on one of axially opposite sides thereof characterized in that the flywheel is an integral one-piece forged structure which is formed by forging a carbon steel blank and which consists of an inner portion having the friction surface and an outer ring gear portion which serves as the ring gear and which is disposed radially outwardly of the inner portion.
The flywheel of the present invention constructed as described above is an integral one-piece forged structure formed by forging a carbon steel blank, for example, a blank of a carbon steel material for machine structural use. Accordingly, the tensile strength of the flywheel at the attaching section of its inner portion is significantly increased, so that the required wall thickness at the attaching section can be accordingly reduced. Therefore, the present flywheel may be designed to have an increased ratio of the weight of the outer ring gear portion to the total weight, and an accordingly increased moment of inertia.
Further, the outer ring gear portion formed by forging has a higher strength owing to a continuous metal structure, than the ring gear of the conventional flywheel, which is formed by hobbing or any other tooth cutting operation.
In addition, the present one-piece flywheel can be manufactured at a reduced cost, with a reduced number of process steps, since the integral one-piece forging structure of the flywheel is formed by forcing of the carbon steel blank, without having to prepare two separate members, that is, an inner body member and a ring gear member, and assemble these two members together, as required to manufacture the conventional flywheel.
In one preferred form of the present flywheel, the outer ring gear portion consists of a toothed section and a base section which are two mutually adjacent axial sections of the outer ring gear portion. The toothed section is formed by forging a cylindrical blank as the blank in an axial direction thereof and has a multiplicity of teeth which project in a radially outward direction of the outer ring gear portion and extend in an axial direction of the outer ring gear portion and which are spaced apart from each other in a circumferential direction of the outer ring gear portion. The teeth of the toothed section are connected to each other by the base section which is adjacent to the toothed section in the axial direction of the outer ring gear portion. In this form of the flywheel, the toothed section of the outer ring gear portion has an increased mechanical strength, in particular, increased toughness, even where the toothed section is hardened. Accordingly, the flywheel exhibits improved durability at a high operating speed.
The toothed section having the multiplicity of teeth is preferably located on the axial side of the flywheel which is remote from the friction surface provided on the inner portion.
In another preferred form of the flywheel of the present invention, the friction surface has a multiplicity of minute recesses or micro holes formed by shot peening after the forging on the carbon steel blank. In operation of the flywheel, the friction surface is brought into frictional contact with a friction member such as a clutch disc. In the presence of the minute recesses formed in the friction surface of the flywheel, there remains a network or matrix of air between the friction surface and the friction member, which makes it possible to minimize an increase of the friction coefficient of the friction surface even at a relatively high operating temperature, thereby reducing the amount of wear of the friction surface and prolonging the expected service life of the friction surface. In this respect, it is noted that the frictional sliding contact of the friction surface and the friction member causes a rise of the temperature at the friction surface. In the conventional flywheel including the inner body member formed of a cast iron, a graphite contained in the cast iron is melted and functions as a lubricant which reduces a rise of the friction coefficient at the high temperature, thereby reducing the amount of wear of the friction surface. In the present flywheel formed of a carbon steel material, the friction surface the temperature of which tends to rise, is desirably subjected to shot peening to form a matrix of minute recesses, which is effective to prevent an excessive rise of the temperature at the friction surface and an excessive increase of the friction coefficient of the friction surface.
It is noted that an excessive rise of the friction coefficient of the friction surface of the flywheel may cause a quick engagement of the friction member with the friction member such as a clutch disc. In this case, the flywheel when used for an engine of an automobile may suffer from the following problems:
a) The clutch of the vehicle cannot be smoothly operated, leading to easy stall of the engine.
b) The power transmission system of the vehicle may be subject to an abrupt change in the load torque acting thereon, leading to reduced service life of the power transmission system.
c) The engine of the vehicle is likely to suffer from a juddering phenomenon at a relatively low operating speed.
d) The clutch disc suffers from an excessive wear, leading to reduced service life thereof.
According to a further preferred form of the flywheel of this invention, the friction surface is hardened prior to the shot peening. For instance, the friction surface is hardened to within a range of about HRC45-47. The hardening of the friction surface is effective to reduce the rate of wearing of the friction surface. The wearing of the friction surface causes an increase in the smoothness (i.e., a decrease in the roughness) of the friction surface, even where the friction surface is initially given the minute recesses. In other words, the hardening of the friction surface maintains the minute recesses in their original shape and size for a prolonged period of time, and thereby maintains a sufficient amount of air between the friction surface and the friction member, to thereby hold the friction coefficient therebetween at a value low enough to prevent an excessive rise of the temperature, which would cause thermal decomposition of the material of the clutch disc. Therefore, the hardening of the friction surface permits the present carbon steel flywheel to have operating durability and service life which are comparable with those of the conventional gray cast iron flywheel.
The second object indicated above may be achieved according to a second aspect of this invention, which provides a process of manufacturing a flywheel which is generally disc-shaped and which includes a ring gear disposed along a periphery thereof and has a friction surface on one of axially opposite sides thereof, characterized by comprising the steps of: (a) forging a carbon steel blank to obtain an intermediate product which is a generally disc-shaped integral one-piece forged structure consisting of an inner portion having the friction surface and an outer ring gear portion which serves as the ring gear and which is disposed radially outwardly of the inner portion; (b) hardening the friction surface of the inner portion of the intermediate product; and (c) shot peening the hardened friction surface to form a multiplicity of minute recesses in the hardened friction surface.
The process of the present invention has the same advantages as described above with respect to the flywheel of the invention, and the advantages described above with respect to the minute recesses formed in the friction surface and the hardening of the friction surface.
The principle of the present invention is particularly applicable to a flywheel which is coaxially connected to the crankshaft of an engine of a motor vehicle and which is a generally disc-shaped member having an outer ring gear portion. However, the principle of the invention is equally applicable to any other rotating member. Where the flywheel is used with the engine crankshaft, a pinion driven by an engine starter motor meshes with the outer ring gear portion of the flywheel.
According to one preferred form of the present process, the step of forging a carbon steel blank to obtain an intermediate product comprises: hot-forging a cylindrical carbon steel blank to obtain a first cylindrical intermediate product having an inner disc portion and an outer annular portion which are integral with each other; and cold-forging the first cylindrical intermediate product to form the outer ring gear portion at the outer annular portion, for thereby obtaining a second cylindrical intermediate product as the generally disc-shaped integral one-piece forged structure consisting of the inner portion and the outer ring gear portion.
However, the generally disc-shaped integral one-piece forged structure of the flywheel may be obtained in a single forging step.
In one arrangement of the above preferred form of the present process, the step of cold-forging the first cylindrical intermediate product comprises moving the first cylindrical intermediate product relative to a gear forming die in an axial direction of the first cylindrical intermediate product, the gear forming die having a multiplicity of tooth forming teeth for forming a multiplicity of teeth of the outer ring gear portion.
The process according to another preferred form of this invention further comprises the step of hardening a surface of the outer ring gear portion. The process may further comprise the step of cutting a plurality of holes through the attaching section of the inner portion, which holes are used for attaching the flywheel to the crankshaft or other rotary member.
The shot peening of the friction surface may be effected with steel particles driven by a blast of compressed air or by a centrifugal force against the friction surface, at a velocity of about 100 m/sec, for example. The steel particles have a diameter or size selected within a range of 40-200 xcexcm and a hardness not lower than that of the carbon steel material of the flywheel (blank). The minute recesses or micro holes formed in the hardened friction surface preferably have a depth of several microns. The shot peening is preferably adapted to give the friction surface a hardened layer having a thickness of about 10-20 xcexcm, which is larger than the depth of the minute recesses. The hardness of the hardened layer is higher than the hardness given by the hardening operation such as induction hardening. The shot peening may be referred to as xe2x80x9cwide peening cleaning (WPC)xe2x80x9d.
The step of hardening the friction surface is preferably effected by induction hardening such as hardening by high-frequency induction heating, prior to the shot peening of the friction surface. The friction surface may be hardened to HRC45-47.